The invention relates to a device for equal-rated parallel operation of inductively coupled inverters, which are connected to a common supply line via network coupling impedances, each inverter being provided respectively with a control circuit which is intended to control its output voltage, to which control circuit a reference voltage is supplied as desired voltage, the frequency of which is derived from the active power taking into account preselected frequency statics and the amplitude of said reference voltage being derived from the reactive power of the relevant inverter taking into account preselected voltage statics.
Devices for equal-rated (democratic) parallel operation of a.c. voltage sources are desired above all for constructing modularly extendable, decentralised electrical supply systems, since they make possible a simple increase in the system power and system reliability (nxe2x88x921 redundancy). It is understood by xe2x80x9cequality of ratingxe2x80x9d that each a.c. voltage source forms the network with a sinusoidal voltage of for example 230 Veff at a frequency of for example 50 Hz and contributes to the supply. In addition to the redundancy resulting therefrom, a simple installation should in addition be possible.
In particular inverters are possible here as a.c. voltage sources which generate a controlled a.c. voltage or can set a prescribed a.c. voltage (normally 50 Hz, 230 Veff). This can concern single- or three-phase inverters, the output voltages of which are controlled by a pulse width control and which obtain their input d.c. voltages, for example from solar cells, fuel cells, batteries or the like. Subsequently, all the a.c. voltage sources which are relevant here are combined in brief with the description xe2x80x9cinvertersxe2x80x9d.
In the case of the devices applied to date for these purposes, a complete equality of rating of the inverters has not always been present. This applies for example to systems which are constructed from a single voltage source and a plurality of current sources. Systems of this type require not only additional lines for the exchange of information, which is undesirable, but are also not redundant because both the failure of the voltage source and also a defective exchange of information leads to a network closedown [1]. The same applies for known systems, in which in fact the network is formed exclusively from inverters operated as voltage sources, but in which these are synchronised by a master voltage source so that a fault in the synchronisation system can lead to network interruption [2].
In addition, it is known to change the frequency and the voltage of inverters in accordance with their active and reactive power with the help of statics which are known from the interlinking (interconnection) network. As a result, a complete equality of rating of the inverters can in fact be achieved but other problems arise during the production thereof.
For example, the operation of inverters with statics which are not network-compatible is known which implies that, as a result of the ohmic coupling of the voltage sources, the active power depends upon the voltage [P(u)] and the reactive power upon the frequency [Q(f)] [3]. Of disadvantage here is the principal-conditioned imprecision of the active power distribution which can be influenced for example also by the connection lines.
Finally, devices of the initially described type are known which operate with network-compatible statics [4]. Network-compatible implies here that, according to FIG. 1, at least two inverters 1 and 2 as in the case of normal interlinking networks are coupled inductively with a supply line 3 and that therefore the active power depends upon the phase xcfx86 and via this upon the frequency [P(f)], and the reactive power depends upon the voltage [Q(u)].
Typically very small network coupling impedances 4 and 5 of for example 0.8 mH are thereby provided as inductances which operate rapidly and are economical.
It is essential for equal-rated parallel operation of inverters in the case of devices of this type initially that
all involved inverters require an identical frequency,
only small voltage difference are permitted between the inverters, and
the phase differences between the inverters must be very small.
In addition, it is a prerequisite that each inverter requires its own voltage- and frequency reference, if additional synchronisation and/or communication lines are intended to be dispensed with. However, a problem resides in the fact that known references (for example quartz oscillators) do not have negligible tolerances for this application case. Ageing effects, faults in the current and voltage detection, temperature dependencies or the like lead to imprecisions with the result that the output voltage of each inverter must be controlled, for the purpose of which only the temporal course of its output voltage uactual and of its current iactual is available.
In the case of the initially described, known device [4], the control of the voltage of each inverter 1, 2 is effected with a control device according to FIG. 2. The main item of such a control device is a unit 6 with two inputs, to which the actual values of the voltage uactual or of the current iactual of the respective inverter, here the inverter 1, are supplied. This unit 6 determines a reference voltage uref, which is determined according to frequency and amplitude, using frequency- and voltage statics, said reference voltage serving in a control circuit 7 for the inverter 1 as command variable or nominal or desired value. By comparing the reference voltage uref with the respective actual voltage uactual of the inverter 1, a signal for controlling the relevant inverter 1 is derived, which normally comprises an adjustment signal for its pulse width.
A disadvantage of the known device resides in the fact that it can be operated only with relatively flat frequency statics which, because of the tolerances of the available quartzes and of other components, lead to difficulties in implementation and hence cannot be converted industrially. On the other hand, if realistic, steeper statics are used, these then lead to power pendulum oscillations between the parallel-connected inverters and hence to instabilities, such as are illustrated in FIG. 3 by way of example.
Starting from the above an object underlying this invention is to configure the device of the initially described type such that comparatively steep statics can be used.
A further object is to configure the device of the initially described type such that hunting and build up of the active power components is effectively avoided even if comparatively steep statics are used.
A further object of this invention is to design the device such that additional synchronisation and communication lines can be avoided.
Yet another object is to propose a device of the initially described type wherein oscillations and build-ups of the active power components are remarkably reduced if steeper statics are used.
These and other objects are solved by the invention in that a value for the phase of the reference voltage is also derived from the active power taking into account a preselected weighting coefficient.
The invention includes the surprising advantage that the instabilities mentioned with reference to FIG. 3 can be entirely avoided by the additional consideration of the phase during the control of the inverters. As is explained subsequently in more detail with reference to a preferred embodiment, there are obtained rather, even with use of comparatively steep frequency or active power statics, stationary active power values even after a short transient process. As a result, the substantial advantage is produced that the active power statics can be established in more or less any manner.
Further advantageous features of the invention are produced from the sub-claims.